In the prior art, respiratory wave signals are obtained through a respiration measuring device by using an impedance-based measuring method. In this method, a high frequency carrier wave signal is applied to the human thoracic cavity by means of an AgCl electrode stuck to a certain position on the body surface of the human during the monitoring of body surface electrocardio-signals. The human thoracic cavity with a constant volume will have a constant basic impedance with respect to the high frequency carrier wave, so when respirations result in the change in the volume of the thoracic cavity, the impedance of thoracic cavity will change slightly. As a result, the respiratory change may be reflected by this slight change in the impedance of thoracic cavity and may be further modulated on the high frequency carrier wave signal. The high frequency carrier wave signal modulated by human respiration may be fed to a respiration amplifying circuit via an electrocardio-cable, and then carrier wave amplification, carrier wave detection and demodulation and respiratory wave amplification will be performed. Finally, a volt-level respiration signal will be obtained. After A/D (analog-to-digital) conversion, a digital respiratory wave signal will be provided, which can be used for respiratory wave feature recognition and respiratory rate computation.
Usually, the respiration rhythm of human is relatively stable. The normal respiratory rate of an adult is 10-30 BPM (Beats per Minute), and the normal respiratory rate of an infant is 30-70 BPM. Therefore, in consideration of abnormal circumstances, the detection range of a respiration detection circuit is usually required to be 8-120 BPM, and sometimes it is required to be up to 150 BPM. The frequency of the respiratory wave corresponding to such range of respiratory rate is about 0.125-2.5 Hz. Due to the individual difference between human bodies, the basic impedance of thoracic cavity of a human is usually about 200-5000 ohm, and the variation of the impedance of thoracic cavity caused by respirations is about 0.3-3 ohm. As a result, the original respiration signal generated by the impedance variation is in the magnitude of tens of microvolts (such as 0.05-0.5 mV).
The process for measuring the respiration signal by means of impedance is prone to undergo interferences, which mainly come from limb movements and cardiac blood-ejection activities causing the variation of the impedance of thoracic cavity of a human. In the respiration measuring process, especially for infants, limb movements cannot be avoided. The variations of thoracic and abdominal impedances caused by limb movements sometimes are sufficient to exceed the slight variation of impedance caused by human respiration. In this case, respiration signals cannot be detected and recognized. Similarly, depending on the differences between individuals, the variation of the impedance of thoracic cavity caused by cardiac blood-ejection activities may also affect respiration signals. It has been found that for some individuals under test, heartbeat activities cause so great interference on respiration signals that Cardiovascular Artifact (CVA) will appear. As a result of such interference, the measuring device wrongly recognizes heartbeat signals as respiration signals, so that the respiratory rate thus computed will be higher than it actually is.
Respiratory rate computation and asphyxia alarm are the two main tasks of respiration measuring. Accurate respiratory rate and accurate asphyxia alarm depend on high recognition rate of respiratory wave. Usually, filtering is performed by lowpass or bandpass filters. Waveform recognition is implemented by a recognizing method based on baseline (i.e., the mean value of the amplitudes during a period of time) threshold or variation threshold. The existence or inexistence of respiratory wave during a certain period of time is determined by comparing the position of the respiratory wave relative to its baseline, and the respiratory rate is computed by an averaging method.
The above method of the prior art is advantageous in that the recognizing process is relatively intuitionistic. However, the shortages of this method are: when there exist limb movement interference and baseline drift caused by it, miss-recognitions of the respiratory wave may appear with this method; wrong asphyxia alarms may be generated when the strength of respiration is unstable; and wrong waveform recognitions may be generated especially when CVA interference exists. In conclusion, this method can't resist various interferences and is insensitive to signal variations, resulting in the inaccuracy and instability of the respiratory rate measurement.